Adsorption of Divalent Metal Ions on a Herbaceous Peat

Transcription

1 Asian Journal of Chemistry Vol. 21, No. 5 (29), Adsorption of Divalent Metal Ions on a Herbaceous Peat HÜSEYIN DIKICI Department of Soil Science, Kahramanmaras Sütçü Imam University 466 Kahramanmaras, Turkey Fax: (9)(344)22348; Tel: (9)(344) hdikici@ksu.edu.tr This study was performed to determine divalent metal ion [Cu(II), Zn(II), Mn(II) and Ni(II)] adsorption characteristics of the herbaceous peat of the Gavur Lake, Turkey. The adsorption data of the metal ions were fitted to both the Langmuir and the Freundlich isotherms. The adsorption capacity of the peat was much higher for Cu (38.5 mg g -1 ) compared with Zn (28. mg g -1 ), Mn (26.4 mg g -1 ) and Ni (24.1 mg g -1 ). A steady increase in equilibrium Ca and Mg ion concentrations were measured as initial metal ion concentrations increased from 1-3 M to 5-3 M in Cu and Zn adsorption experiments indicating that ion exchange was an important mechanism in the adsorption. The kinetic study revealed that the adsorption followed the pseudo-second-order model and the interaction between the metal ions and the peat was very fast, especially for Cu. The ph study revealed that Cu adsorption was not affected in the ph range between 3 and 5, but the adsorption of Zn, Mn and Ni was lower at ph 3 compared with ph 4 and 5. Key Words: Adsorption, Kinetics, Peat, Copper, Zinc, Manganese, Nickel. INTRODUCTION There is an increasing environmental concern of heavy metal loading to water resources from industrial wastewater discharges. High levels of the heavy metals such as copper, zinc, manganese and nickel are harmful for many life forms and cause brain, kidney, lungs, liver and central nervous systems failure in humans 1. Copper is released to environment via mining, metallurgical, fibre production, pipe production and metal plating industries. Zinc is mainly used in galvanization processes; nickel is important for electroplating and manganese finds its use in dry cell batteries, glass, ceramics, paint, varnish, ink, dyes, match and fire works 2. Many processes exist for the treatment of wastewaters including ion exchange, precipitation, phyto-extraction, ultra-filtration, reverse osmosis and electro-dialysis and among these methods ion exchange seems most promising due to easy handling if effective and low cost sorbents are used 3. The adsorption of metal ions from aqueous solutions has been studied using more than 7 different sorbents 4 such as perlite 5, vermiculite 6, kaolinite 7, tree fern 8, palm kernel fibre 9, carbon aerogel 2, Nipah palm shoot biomass, clinoptilolite 3, fly ash 11 and peat 12,13. Peat has polar functional

2 Vol. 21, No. 5 (29) Adsorption of Divalent Metal Ions on a Herbaceous Peat 3335 groups such as aldehydes, ketones and acids 13. Metal ion adsorption by peat may take place via several meachanisms such as ion-exchange, surface sorption, chemisorption, complexation and sorption-complexation and it is believed that ionexchange is the most prevalent mechanism 12. The metal adsorption capacity of peat depends on many factors such as ph, temperature, contact time, peat doze, botanical origin of peat, metal ion and initial metal ion concentration 13,14. This study was performed to determine Cu(II), Zn(II), Mn(II) and Ni(II) sorption characteristics of the herbaceous peat of the Gavur lake, Turkey. EXPERIMENTAL The herbaceous peat used in this study was obtained from the Gavur lake Peatland of Turkey. The peatland, which is surrounded by Miocene aged limestone and Paleozoic schist and serpentines formed on Holocene aged lake deposits in a closed watershed 15 on the Antakya-Kahramanmaras Graben, ca m asl, 3 km south of the Kahramanmaras city, Turkey. The area was drained by the State Hydraulic Works in the late 195s using artificial canal networks which flow into the Aksu and Ceyhan rivers. The peat sample was oven dried at 5 ºC and grounded through a µm sieve. The soil sample was analyzed for organic matter by wet oxidation 16, ph 17 and CaCO The studied peat material has 53.6 % organic matter, neutral ph and 4.3 % total carbonate content (Table-1). TABLE-1 SOME PHYSICAL CHARACTERISTICS OF THE PEAT MATERIAL Organic matter (%) 53.6 ph 6.97 Total carbonate (%) 4.3 Adsorption experiment: Copper, Zn, Mn and Ni stock solutions were prepared using analytical grade CuSO 4.5H 2 O, ZnCl 2, MnCl 2.4H 2 O and NiCl 2.6H 2 O. A.5 g soil sample was weighed into a 125 ml Erlenmeyer flask and 5 ml solution containing between 1-3 M and 5-3 M concentrations of the studied metal ions ( µg ml -1 for Cu) were added into each flask. The adsorption experiments were run in triplicates. Initial ph of the solutions was adjusted to 5, except the ph study, using 1-3 M solutions of either H 2 SO 4 or NaOH. The flasks were shaken for 3 h at 21 ± 1 ºC and filtered. Equilibrium metal ion concentrations were determined using an atomic absorption spectrometer (Perkin-Elmer 31). The amount of metal ion adsorbed by peat was calculated by the difference between the initial and equilibrium concentrations of the solutions. In order to show the effect of cation exchange in the adsorption process, the displacement of Ca and Mg ions in Cu and Zn experiments was measured using an atomic absorption spectrometer (Perkin-Elmer 31).

3 3336 Dikici Asian J. Chem. The effect of solution ph on metal ion adsorption was studied in the ph range between 3 and 5. Initial ph of the solutions was adjusted to 3, 4 and 5 using 1-3 M solutions of either H 2 SO 4 or NaOH and the flasks were shaken for 3 h at 21 ± 1 ºC and filtered. The concentrations of the metal ions in equilibrium solutions were determined as mentioned above. Kinetics of the adsorption were studied by shaking a.5 g peat sample with 5 ml solutions of 2-3 M and 4-3 M metal ions in a 125 ml Erlenmeyer flask. Samples were withdrawn at suitable time intervals up to 3 h, filtered and analyzed for the studied metals with an atomic absorption spectrometer (Perkin-Elmer 31). Theoretical: The metal adsorption by the peat was modeled using the Langmuir and Freundlich isotherms. The Langmuir model implies that the monolayer adsorption of metal ions on sorbent surface occurs, energy of sorption for each ion is same and not dependent on surface coverage and adsorption takes place only on localized sites 13. The Freundlich model, however, assumes that the energy of adsorption decreases logarithmically as surface coverage increases due to surface heterogeneity. Langmuir isotherm: C e /q e = 1/kq max + C e /q max where q e is the amount of adsorbed metal ion by the peat (mg g -1 ), C e is the equilibrium solution concentrations (mg L -1 ), q max is the adsorption maximum or the adsorption capacity (mg g -1 ) and k is a constant related to the sorption energy (L mg -1 ). If adsorption conforms to the Langmuir equation, a plot of C e /q e vs. C e will form a straight line with a slope 1/q max and intercept 1/k q max. Freundlich isotherm: log q e = log K f + 1/n log C e where K f and 1/n are constants of adsorption capacity and adsorption intensity, respectively. When Freundlich equation applies, log q e versus log C e will yield a straight line with a slope 1/n and intercept K f. Kinetics of the adsorption experiment was modeled using the pseudo-second order kinetics 19 : t/q t = 1/k 2 q e2 + (1/q e ) t and initial sorption rate calculated with the following formula: 2 h = k 2 q e where h is the initial adsorption rate (mg g -1 min -1 ), k 2 is the rate constant (g mg -1 min -1 ), q t is the amount of the metal ions on the surface of the adsorbent at any time (mg g -1 ) and t is time (min). If the pseudo-second-order model fits the data, t/q t versus t will have a straight line with a slope 1/q e and intercept 1/h.

4 Vol. 21, No. 5 (29) Adsorption of Divalent Metal Ions on a Herbaceous Peat 3337 RESULTS AND DISCUSSION Sorption isotherms: The herbaceous peat adsorbed 96 % of the Cu, Zn and Ni ions and 86 % of Mn ions at 1-3 M initial solution concentrations and the removal efficiency was similar for Cu (97 %) and lower for Zn (77 %), Mn (68 %) and Ni (75 %) at the highest initial solution concentrations (5-3 M). Figs. 1-4 show the plots of the equilibrium solution concentrations versus the metal ions sorbed. The adsorption data of the metal ions were fitted to both the Langmuir and Freundlich isotherms (Table-2). Figs. 5 and 6 show Langmuir and Freundlich isotherms for Cu 2+ sorption on to the peat. Langmuir equation better explained the adsorption of the metal ions on the peat compared with the Freundlich equation based on the coefficient of determination (R 2 ) values (Table-2). Several studies have shown that peat-metal ion adsorption is most effectively modeled with the Langmuir isotherm 12,13. The adsorption capacity (complete monolayer adsorption) of peat was much higher for Cu (38.5 mg g -1 ) compared with the other metals ( mg g -1 ). The adsorption maximum of Zn, Mn and Ni was similar and the affinity of the peat for these metals decreased in the order of Zn, Mn and Ni, respectively (Table-2). Similar order of metal adsorption by peat reported elsewhere 12. The metal adsorption capacity of peat depends on many factors such as ph, temperature, contact time, peat dose, botanical origin of peat, metal ion and initial metal ion concentration 13,14. Ho and McKay 2 reported that the copper adsorption capacity of the eutrophic sphagnum moss peats is in the range of -3 mg g -1 and that of the oligotrophic peats is generally between 1 and 15 mg g -1 based on their literature review. Copper adsorption capacity of peat materials in the similar range was reported elsewhere 14,2-22. qe (mg g -1 ) Fig. 1. Sorption isotherm of Cu 2+ Fig. 2. Sorption isotherm of Zn 2+ qe (mg g -1 ) qe (mg g -1 ) Fig. 3. Sorption isotherm of Mn 2+ Fig. 4. Sorption isotherm of Ni 2+ qe (mg g -1 )

5 3338 Dikici Asian J. Chem. TABLE-2 LANGMUIR AND FREUNDLICH ISOTHERM CONSTANTS FOR THE METALS SORBED ON THE PEAT Element Langmuir Freundlich q max k R 2 K f 1/n R 2 Cu Zn Mn Ni *q max (mg g -1 ), k (L mg -1 ), K f and 1/n are constants..3 Ce/qe (g L -1 ) log qe (mg g -1 ) log Fig. 5. Langmuir sorption isotherm of Fig. 6. Freundlich sorption isotherm of Cu 2+ ions Cu 2+ ions The herbaceous peat material of this study has a neutral ph (6.97) and Ca and Mg are the two important exchangeable cations at this ph 15. One of the reasons for the relatively high metal adsorption capacity of the Gavur Lake peat can be its high Ca and Mg contents on the exchange sites. Since significant correlation was found between the total exchangeable Ca and Mg contents of peats and their heavy metal adsorption capacity 12. It was also reported 23 that heavy metals can easily displace Ca 2+ compared to H +. Displacement of Ca and Mg ions with metal ion adsorption was also studied in Cu and Zn experiments (Table-3). A steady increase in equilibrium Ca and Mg ion concentrations were measured as initial metal ion concentrations increased from 1-3 M to 5-3 M (Table-3). This finding, therefore, shows that ion exchange is an important mechanism in both Cu and Zn adsorption. The total displaced concentrations of Ca and Mg ions were very similar in both Cu and Zn experiments suggesting that ion exchange was not the only mechanism for the adsorption of Cu, since the peat has much higher adsorption capacity for Cu (38.5 mg g -1 ) compared to Zn (28. mg g -1 ). Chen et al. 24 found that the amount of released cations was less than the sorbed Cu ions. They explained this finding with the existence of another mechanism for Cu adsorption in addition to ion-exchange. Metal ion adsorption by peat may take place via several mechanisms such as ionexchange, surface adsorption, chemisorption, complexation and sorption-complexation

6 Vol. 21, No. 5 (29) Adsorption of Divalent Metal Ions on a Herbaceous Peat 3339 TABLE-3 DISPLACEMENT OF Ca AND Mg IONS WITH INCREASING CONCENTRATIONS OF METAL IONS Cu Experiment Zn Experiment C i (Cu or Zn) Ca Mg Ca Mg mg g M M M M M and it is believed that ion-exchange is the most prevalent mechanism 12. Bloom and McBride 25 reported that peat binds most divalent cations as outer-sphere complexes or by electrostatic forces with the exception of Cu, which is bound as inner-sphere complexes. The electronic properties of Cu result in formation of strong innersphere metal surface complexes and a very strong affinity for soil organic matter 26. Freundlich K f values, which is a measure of adsorption capacity ranged in the decreasing order of Cu >> Zn > Ni > Mn. The adsorption intensity values determine whether the adsorption is favourable over the entire concentration range (1/n «1) or favourable at high concentrations and not on low concentrations (1/n > 1) 27. The adsorption intensity constants were.42,.39,.57 and.38 for Cu, Zn, Mn and Ni, respectively, indicating activated adsorption of the metals on the peat. Chen et al. 28 reported that the adsorption intensity (1/n) is related to K f in the Freundlich equation as part of the power factor and variations in intensity constant will yield exponential change in K f. Therefore, higher 1/n value of Mn compared with that of the other metals explains why Freundlich K f values of the metals did not rank similar to the Langmuir q max values of the metals (Table-2). Kinetic study: Kinetics of the adsorption experiments were modeled using the pseudo-second-order model 19. The reaction between the metal ions and the peat was very fast and the system reached equilibrium within.5 h. The parameters of the pseudo-second-order model was calculated by regressing t/q t versus t for the initial metal concentrations of 2-3 M and 4-3 M (Table-4). The kinetic data TABLE-4 PSEUDO-SECOND-ORDER MODEL PARAMETERS OF THE METALS AT 2-3 M AND 4-3 M INITIAL SOLUTION CONCENTRATIONS Element 2-3 M 4-3 M * q e h k 2 R 2 q e h k 2 R 2 Cu Zn Mn Ni *q e (mg g -1 ), h (mg g -1 min -1 ), k 2 (g mg -1 min -1 ).

7 334 Dikici Asian J. Chem. showed a good fit to the pseudo-second-order model (Fig. 7). Equilibrium metal adsorption (q e ) of the peat at 2-3 M and 4-3 M initial solution concentrations were in the following order Cu > Zn > Ni > Mn. The affinity of the metals for the peat at these concentrations was similar to the rank of the Freundlich K f values. Initial adsorption rate (h) of Cu ions (169.4 mg g -1 min -1 ) was much higher than that of the other metals at 2-3 M initial solution concentrations. The adsorption rate constant (k 2 ) also followed the same trend. The initial adsorption rates of Cu and Mn decreased and that of Zn and Ni increased as initial metal ion concentration increased from 2-3 M to 4-3 M (Table-4). The adsorption rate constants of the studied metals, however, decreased with an increase in initial metal ion concentrations. Negative relationship between the rate constant and initial metal ion concentration was also reported elsewhere 29. t/qt (min g mg -1 ) Time (min) Fig. 7. Pseudo second-order kinetics of Cu 2+ at 2-3 M ( ) and 4-3 M ( ) initial solution concentrations Effect of solution ph: Solution ph is an important factor affecting the capacity of peat to hold cations and optimum ph for adsorption 12 is between 3.5 and 6.5. The effect of solution ph on the metal adsorption was studied for solution ph values 3, 4 and 5 at 2-3 M metal ion concentrations. The equilibrium time was 3 h at 21 ± 1 ºC. Fig. 8 shows that Cu adsorption was not affected in this ph range but the adsorption of Zn, Mn and Ni was lower at ph 3 compared with ph 4 and 5. Similar adsorption of the metals at ph 4 and 5 was probably due to buffering effects of the peat and at ph 3 high concentrations of H + ions lowered metal adsorption by ion exchange. Ringqwist and Öborn 14 reported that the solution ph has minor effect on Cu adsorption on peat but it was one of the main factors affecting Zn adsorption. Conclusion (1) The adsorption of the metal ions on to the peat fitted both Langmuir and Freundlich models and the Langmuir model better explained the adsorption. The adsorption capacity of the peat followed the order Cu > Zn > Mn > Ni. (2) The peat

8 Vol. 21, No. 5 (29) Adsorption of Divalent Metal Ions on a Herbaceous Peat ph 3 ph 4 ph 5 Sorption (mg g -1 ) Fig. 8. Cu Zn Mn Ni Effect of initial ph on the sorption of the metal ions for 2-3 M initial solution concentrations removed % of the metal ions at the lowest initial concentrations and between % of the metal ions at the highest initial solution concentration and Cu was the most efficiently removed metal ion in both low and high initial solutions (96 and 97 %). (3) The kinetic study revealed that the adsorption followed the pseudosecond-order model and the interaction between the metal ions and the peat was very fast and equilibrium was reached within.5 h. (4) Cu adsorption was not affected in the ph range between 3 and 5 but the adsorption of Zn, Mn and Ni was lower at ph 3 compared with ph 4 and 5. REFERENCES 1. R.B. Martin, Metal Ions in Biological Systems, Concepts on Metal Ion Toxicity, Marcel Dekker, New York and Basel, p. 21 (1986). 2. A.K. Meena, G.K. Mishra, P.K. Rai, C. Rajagopal and P.N. Nagar, J. Hazard. Mater., 122, 161 (25). 3. M. Sprynskyy, B. Buszewski, A.P. Terzyk and J. Namiesnik, J. Colloid Interface Sci., 34, 21 (26). 4. S.E. Bailey, T.J. Olin, R.M. Brica and D.D. Adrin, Water Res., 33, 2469 (1999). 5. T. Mathialagan and T. Viraraghavan, J. Hazard. Mater., 94, 291 (22). 6. T. Mathialagan and T. Viraraghavan, Sep. Sci. Technol., 338, 57 (23). 7. O. Yavuz, Y. Altunkaynak and F. Guzel, Water Res., 37, 948 (23). 8. Y.S. Ho, Water Res., 37, 2323 (23). 9. Y.S. Ho and A.E. Ofomaja, J. Hazard. Mater., 12, 157 (25).. D. Wankasi, M.J. Horsfall and A.I. Spiff, Electronic J. Biotechnol., 9, 587 (26). 11. I.J. Alinnor, Fuel, 86, 853 (27). 12. P.A. Brown, S.A Gill and S.J. Allen, Water Res., 34, 397 (2). 13. Y.S. Ho and G. McKay, Water Res., 34, 735 (2). 14. L. Ringqvist and I. Öborn, Water Res., 36, 2233 (22). 15. H. Dikici and C.H. Yilmaz, J. Environ. Qual., 35, 866 (26). 16. D.W. Nelson and L.E. Sommers, Methods of Soil Analysis: Chemical Methods, Part 3, Madison, WI, p. 961 (1996).

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